l Couldnt Add a New Thumbnail

Hi all,
Im using whitedeo theme. But l didnt change default thumbnail pictures. In funtion.php:

function get_thumb() {
global $post, $posts;
$first_img = '';
ob_start(); ob_end_clean();
$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
$first_img = $matches [1] [0];
if(empty($first_img)){ $first_img = "/wp-content/themes/whitedeo/images/nothumb.jpg"; }
return $first_img;
}


I see /wp-content/themes/whitedeo/images/nothumb.jpg alltime. How can l send image url to $first_img variable. Im waiting your help. And l want use this theme. Show me the way plz.
Note: Features are using other pic. And l didnt change it too 🙁
Thanx for help…